In a number of industrial work environments workers are at risk of being exposed to a variety of hazardous environmental substances such as toxic or highly combustible gases, oxygen depleted environments, or radiation, etc. that pose a serious threat to worker safety. In order to keep workers safe, specialized environmental monitoring devices are used to alert workers of dangerous changes in their immediate environment.
Current practice involves using fixed point monitoring devices that monitor the environment around where they are deployed or portable monitoring devices that are carried by the workers to monitor their immediate vicinity. Fixed point monitoring devices are typically used around potential hazard locations such as confined spaces to warn workers of the environment before they enter. Portable monitoring devices are often used for personal protection. These monitoring devices may have a single sensor to monitor one specific substance or multiple sensors (typically two to six) each monitoring a distinct substance.
Given that these environmental monitoring devices are life critical, it is important the device functions properly and accurately. Current practice involves periodic bump testing and calibration of monitoring devices to guarantee proper functioning. Bump tests involve exposing the monitoring device to a measured quantity of gas and verifying that the device responds as designed, i.e. it senses the gas and goes into alarm. Calibration involves exposing the device to a measured quantity of gas and adjusting the gain of the sensors so it reads the quantity of gas accurately. The purpose of calibration is to maintain the accuracy of the monitoring device over time.
Current best practice followed by leading manufacturers of environmental monitors recommends bump testing the monitoring device before every days work and calibrating the device once at least every thirty days. While a number of manufacturers sell automated docking stations that automatically perform calibration and bump testing when a monitoring device is docked, there are still a number of disadvantages to the current practice.
A fixed bump and calibration policy, such as currently practiced, does not take into account the actual state of the sensors or the environmental monitoring device. Such a fixed policy (bump test every day and calibrate every thirty days) by its very nature is a compromise that is too stringent in many cases and too liberal in many others.
Given that the docking operation requires the user to bring the monitor to a central location, which typically is outside the work area, to perform the bump test and calibration, there is value in minimizing/optimizing this operation as much as possible without compromising safety.
Threshold limit values (TLV), namely the maximum exposure of a hazardous substance repeatedly over time which causes no adverse health effects in most people is constantly being reduced by regulatory authorities as scientific understanding and evidence grows and we accumulate more experience. Often these reductions are quite dramatic as in the case of the recent (February 2010) reduction recommended by the American Congress of Governmental Industrial Hygienists (ACGIH) for H2S exposure. The ACGIH reduced the TLV for H2S from a time weighted average (TWA) of 10 ppm to 1 ppm TWA averaged over eight hours. The effect of such reductions puts a premium on accuracy of measurements. Current practice of a fixed calibration policy, such as calibrate every thirty days, may not be enough to guarantee the level of accuracy to meet the more stringent emerging TLV's. While a blanket reduction in the frequency of the calibration interval, i.e. from thirty days, will help to improve accuracy, it would add significant cost to the use and maintenance of the environmental monitoring devices.
One solution to this problem, pursued by some, is to use newer and more advanced technology sensors with a higher degree of accuracy and tolerance to drift that minimize the need for calibration and bump testing. While there certainly is value in this approach, the cost of these emerging sensor often preclude its widespread use, particularly in personal monitoring applications where a large number of these monitors need to be deployed.
For all the aforementioned reasons there is value in developing monitors that use current low cost sensor technologies while still meeting emerging TLV regulations and allow for a more adaptive calibration/bump policy that takes into account the state of the sensors and monitoring devices.